Numerous video data compression techniques are already known. These include various video encoding techniques, for example those implementing video compression standards coming from the MPEG (MPEG-1, MPEG-2, MPEG-4 part 2, etc) organization or from the ITU-T (H.261, . . . , H.264/AVC) organization, use a block-wise representation of the video sequence. Thus, according to the H.264 technique, each image can be sub-divided into slices which are then sub-divided into macroblocks. Each macroblock is then sub-divided into blocks. A block is constituted by a set of pixels.
An example of a slice formed by nine macroblocks 111 to 119 is illustrated schematically in FIG. 1. During the encoding and decoding operations, the slices are scanned macroblock by macroblock along a path 12 defined systematically or specified by descriptive elements planned for this purpose, that are present in the stream.
The approach is generally the same for the constituent blocks of a macroblock. A macroblock may be formed, in particular, by 4×4 or 16×16 blocks. According to the prior art techniques, these macroblocks and blocks are encoded by intra-image or inter-image prediction.
Here below, we consider groups of blocks, the term “group” can correspond especially to an image or a slice constituted by a set of macroblocks or to a macroblock, i.e. a set of blocks. The term “block” can be then applied respectively to a macroblock or to a block according to the terminology of the H.264 standard.
According to the H.264 technique, a block may be decoded by:                a temporal prediction, i.e. a reference to a reference block belonging to one or more other images; and/or        a prediction known as a “spatial” prediction as a function of the neighboring blocks of the current image.        
In the latter case, the prediction can be done only from blocks that have been previously encoded.
Thus, in the example of FIG. 2, if we consider the scan 12 of FIG. 1, the block 115 can be predicted only as a function of the previous blocks 111 to 114. The following blocks 116 to 119 which have not been encoded cannot be taken into account.
As illustrated in FIGS. 3A and 3B, in a group of blocks, it is possible to distinguish two zones 31 and 32, generally called respectively a causal zone 31 and an anti-causal zone 32. The causal zone of a group of blocks is the zone of the image comprising the blocks situated before the current block in the direction of scanning of the blocks in the image.
In the H.264 encoder, the direction of scanning of the blocks in the image also corresponds to the direction of scanning of encoding and decoding of the blocks, as well as the direction of scanning of the writing and reading of the encoded data of the blocks in the stream. Here below, the term “causal zone” denotes the zone corresponding to the blocks situated before the current block in the direction of scanning of the writing and reading of the blocks in the stream. The anti-causal zone is the zone of the image comprising the blocks situated after the current block in the direction of scanning of writing or reading of the blocks in the stream.
The prior-art encoding techniques and especially the H.264 encoders use only the blocks of the causal zone for the prediction of a current block since the blocks of the anti-causal zone are not yet encoded or decoded, and are therefore not available for the prediction.
Several types of scans have been defined, as such as the one illustrated in FIG. 1, generally known as a raster Scan or the spiral Scan which proposes the scanning of a group of blocks from its center in going towards the edges by a spiral-shaped scan.
The H.264/AVC standard has defined different patterns for each slice. However, just as in the case in the previous techniques, the scan slice is predefined and leads to distinguishing blocks that can be used for the prediction (causal zone) and blocks that are not usable (anti-causal zone).
The predictions associated with a block may be of various types.
Thus, in the H.264/AVC encoding standard, a texture prediction is proposed along five possible directions of prediction, in the case of blocks sized 4×4. For each block pixel, the prediction is obtained by extending the last line or column of the reference block in a combination of pixels of the last row and/or column.
The invention also proposes, in this encoding technique, a motion prediction. The reference blocks used in the inter prediction or temporal prediction are identified by means of motion vectors, encoded in the stream, in using a prediction by vectors of blocks of the causal and temporal neighborhood. For example, a vector of a block to be encoded may be predicted by computing the median vector from a vector of the top, top left-hand, and left-hand of the block to be encoded.
There is provision also for a prediction known as shifted intra-prediction using already reconstructed blocks of the image as the basis of prediction of a block to be encoded. A reference block that is the closest to the block to be encoded is sought in the part of the image to be encoded that is already reconstructed. The difference between the values of the pixels of the target reference block and the block to be encoded is minimized, and the target reference block is identified in the decoder by means of the encoding of an intra-image motion vector indicating the shift relatively to the block to be encoded to retrieve the reference block.
One drawback of these different prediction techniques, which rely on the use of a write and read scan, of the blocks in the stream or signal produced is that the possible prediction references are limited to the causal zone. Indeed, in certain situations, it would be more efficient for reasons of similitude between the blocks for example to use a prediction base that is a block present in the anti-causal zone.
This however is not possible in the prior art because the blocks of the anti-causal zone have not yet been processed and are therefore not available for the prediction.
It must be noted that, for those skilled in the art, this situation is unavoidable as can be seen from the term “anti-causal zone” and not as a drawback liable to be corrected.